Energy stores for storing and delivering electrical energy are of great importance for example for many mobile applications. While the storage capacity of modern energy stores for storing electrical energy is sufficient for the operation of relatively small devices, such as cell phones, portable computers, etc., energy stores for storing electrical energy for larger applications, such as for example electrically driven motor vehicles, still have shortcomings that stand in the way of their commercially successful use. In particular, the storage capacity of the batteries used does not yet meet the desired requirements. Although, for example, lithium-ion batteries achieve good results for use in cell phones or computers for instance, they are only suitable to a limited extent for applications requiring a large amount of energy, such as for example electrically operated motor vehicles. The storage capacity of the lithium-ion batteries represents a limiting factor here, for example for the range of an electric motor vehicle.
In particular in the motor vehicle sector, there are also known systems in which the energy necessary for propulsion is stored chemically in the form of hydrogen. By means of a fuel cell, the hydrogen is then converted into electric current, by which the engine can be driven. For such technology, however, the setting up of a network of filling stations for hydrogen is necessary, which makes the introduction of this technology expensive, in particular in view of the high safety requirements for the filling stations because of the risk of explosion.
More recently, batteries in which the electrical energy is stored in the form of a reversible redox process between a metal and oxygen have also additionally been considered. The structure of such a battery corresponds approximately to a fuel cell with a solid electrolyte. They are therefore also referred to as solid electrolyte batteries or metal-air batteries. The electrolyte is arranged between two electrodes, one of which is an air electrode, which consists of a material that splits the atmospheric oxygen and conducts the oxygen ions thereby produced to the electrolyte. The electrolyte is likewise produced from a material that can conduct oxygen ions. Arranged on the opposite side thereof from the air electrode is the second electrode, which consists of a metal or metal oxide to be oxidized and reduced. The battery is discharged by the metal being oxidized by means of oxygen ions from the atmospheric oxygen and is charged by the metal being reduced and giving off oxygen ions when a voltage is applied, the oxygen ions then migrating through the electrolyte to the air electrode, from where they are given off to the surroundings as molecular oxygen. This process is schematically represented in FIG. 1, in which the upper half represents the discharging process and the lower half represents the charging process. In this figure, the reference numeral 1 denotes the battery, the reference numeral 3 denotes the air electrode, the reference numeral 5 denotes the metal or metal oxide, the reference numeral 7 denotes the electrolyte, the reference numeral 9 denotes a load that is supplied with current when the battery is discharged, and the reference numeral 11 denotes a power source, which is used when charging the battery. The electrolytes used in the batteries display a highly selective-oxygen-ion conduction, but require relatively high operating temperatures of typically 600° C. or more.
Recent developments in the area of solid electrolyte batteries have led to the second electrode no longer being used itself as a storage medium, providing instead an additional storage medium that is formed from the redox pair. An additional, fluidic redox pair is then provided, implementing the mass transfer between the second electrode on the one hand and the storage medium on the other hand. This allows the power density and the capacity of the solid electrolyte batteries to be increased.
The previous design of the storage elements is based on a skeletal structure with a high open porosity. To reduce the sintering tendency at the operating temperatures prevailing in the solid electrolyte batteries, iron particles known as ODS iron particles (ODS: Oxide Dispersion Strengthened) are used.